1. Field of the Invention
This invention relates to, among others, graphitic molded articles or moldings comprising, essentially, graphite powder and mesophase-containing pitch and to processes for producing the same.
2. Technical Background of the Invention
In the prior art, in the production of graphitic moldings using graphite as the base material, methods in which coal tar pitch, phenol resin, furan resin, etc., are used as the binder for graphite have been known.
When coal tar pitch, phenol resin, etc. are used as the binder, because the melting point of the binder is as low as around 100.degree. C., heating and kneading with the base material and molding can be performed relatively easily. On the other hand, however, these binders are disadvantageous in that they cause a low carbonization yield such as 50 to 60 wt. % in carbonization at up to about 1,000.degree. C., in that the amount of shrinkage accompanying carbonization is great, and also in that shrinkage strain is caused when high speed carbonization is carried out. Further, when a green molding with a complicated shape is carbonized, a problem arises in that complicated deformations due to the difference in amount of shrinkage will occur in the molding obtained.
Now, a phosphoric acid type fuel cell, on which research for practical application is being undertaken on a large scale in United States of America and Japan, employs hot phosphoric acid at around 200.degree. C., and hence its electrode substrates and gas separators are both constituted of carbon materials with low electric resistance and high resistance to hot phosphoric acid. In this connection gas impermeability is a very important requirement for the gas separators to be used in fuel cells.
3. Prior Art and its Problems
As a gas impermeable carbon material to be used for a gas separator in a fuel cell, a glassy carbon material has hitherto been proposed. For example, in Japanese Patent Laid-Open Publication No. 150275/1983, an example of using a glassy carbon alone is described, and in Japanese Patent Laid-Open Publication No. 72273/1982, carbonization (1,000.degree. C.) of a green molding of graphite and phenol resin is carried out for a long period of 168 hours with application of a certain load on the molding, and further the temperature is elevated up to 2,800.degree. C. over 48 hours to obtain a graphitized separator. Also, in Japanese Patent Laid-Open Publication No. 127377/1984, a material with a thickness after calcination of 0.1 to 1 mm, obtained from furan resin to which ultra-fine carbon black powder has been added and admixed according to necessity by molding to a desired thickness, and then curing at room temperature, is placed between graphite plates or the like of excellent thermal conductivity with form-retaining characteristics and is carefully calcined for uniform calcination to obtain a glassy carbon thin plate.
However, according to any of these processes, no epoch-making countermeasure has been adopted to cure the problem of deformation of the molding, which is due to volume shrinkage accompanying the characteristic of a carbonization yield of 50 to 60 wt. % of the phenol resin or the furan resin, which is the starting material for glassy carbon for carbonization up to about 1,000.degree. C.
Also, concerning gas separators, in addition to the flat plate as disclosed in the above techniques, the so called ribbed separator, having divided gas channels crossed at right angles on both sides of the plate, has also been proposed. However, the difficulty in producing in one step a gas separator having such a complicated shape by carbonization of a green molding with the use of graphite as the base material and a phenol resin as the binder is suggested by Japanese Patent Laid-Open Publication No. 140977/1983. That is, according to said Publication, deformation or warping of the plate will occur because of the grooves constituting a large number of gas channels in such a complicated shape as the ribbed separator, and hence a process for carbonization is disclosed in which the coefficient of shrinkage of the green molding during carbonization is previously measured and powders with various formulations of graphite and phenol for respective potions are placed in the mold before carbonization. However, in another application by the same applicant (Japanese Patent Laid-Open Publication No. 119163/1983), the poor yield of the powder filling method is pointed out, and a method in which gas separators are made into flat plates is proposed. Also, according to Japanese Patent Laid-Open Publication No. 150275/1983, in order to obtain a ribbed separator with a furfuryl alcohol resin alone according to a certain process, there is disclosed a method in which a mold for centrifugal molding is formed in conformity with the degree of shrinkage.
However, the gas separator obtained by the technique disclosed in Japanese Patent Laid-Open Publication No. 72273/1982 cannot but be expensive, while according to the methods disclosed in Japanese Patent Laid-Open Publication Nos. 140977/1983 and 150275/1983, since the amount of shrinkage which changes continuously and finally is determined approximately, the dimensional precision will become unstable, and the work must inevitably become complicated.
For practical application of phosphoric acid type fuel cells, there is a great need to overcome the various technical difficulties concerning gas separators as described above.